uwb: wrong sizeof argument in mac address compare

In the __uwb_beca_find_bymac function the sizeof returns
the length of a pointer and not the data it is referring
to. Hence only the first 4 bytes were compared, not the
entire 6 bytes MAC address. Now the sizeof takes struct
uwb_mac_addr as argument.

Signed-off-by: Frank Leipold <frank.leipold@eads.net>
Signed-off-by: David Vrabel <david.vrabel@csr.com>